the SB8200 should be good past though speeds as its docsis 3.1 and has gigabit ports. 

You might have been upgraded to 1.2gig speeds which is technically isn’t compatible with since it uses 1 gig ports. Xfinity has been doing the sneaky and saying devices aren’t compatible for that reason. but it’ll still work. 

Thanks for this answer plemans,

My Xfinity account page says I have and am paying for 800 mbps.  So, is there any way of knowing in my existing configuration with the SB8200 whether I actually have 1.2 gig?

what speeds do you actually get? 

I have a Google mesh network in the house.  When I measure speed with the Google home app, it gives me 897 mbps down and 25 up.  However, when I use the Speedtest app it shows only 320 down and 24 up.